This invention relates generally to tape systems for data storage and retrieval. More specifically, the invention relates to methods of improving data storage, and, specifically, the writing operation. The invention provides methods for minimizing the effect of off-track errors.
Various computer data storage tape systems are well known. Tape systems are commonly used for data protection backup of computer files, and for dumping entire hard drive contents to a tape for backup and archiving. One category of such systems is the linear tape system, a subset of which subscribes to the xe2x80x9cLTO standardsxe2x80x9d (Linear Tape Organization standards). The LTO standards define requirements for hardware and software to enable universal use of standard tapes on LTO equipment produced by various manufacturers. LTO tape 10 is approximately xc2xd inch in width. Eight channel LTO tape has 384 tracks, readable and writeable by a head 12 with eight read/write elements 14.
Schematics and information describing LTO technology are available in patent literature and at the website ltotechnology.com. The tape and writing head of a conventional LTO system is schematically portrayed in FIG. 1. The eight read/write elements 14 in a conventional LTO system may simultaneously write or read eight tracks at a time all along the length of the tape 10, which is typically 600 meters long. After reaching the end of the tape 10, the head 12 is moved slightly, and the tape 10 is run in reverse. The position of the head 12 on the tape 10 is controlled in relationship to servo bands 16 on the tape that each contains six sub-bands. Also, the head 12 has two possible positions for each of the sub-bands within each of the servo bands. As a result, the tape has a total of (8xc3x974xc3x976xc3x972)=384 tracks for reading and writing.
Conventional linear tape systems have a maximum write and read speed, typically in the range of 2-8 meters/second, and, in the preferred embodiment herein, about 4.1 meters/second (m/s). The linear tape travels at this speed through guides between two spools. Due to external vibrations, internal inconsistencies in tape, spool, or guide manufacture, and inconsistencies in the wrapping of tape around itself on the spool, the tape sometimes wanders transversely, relative to the writing head, so that centers of the tracks move transversely from the writing elements. This is called xe2x80x9cgoing off-track.xe2x80x9d These off-track events typically repeat themselves and frequently grow to a magnitude that results, if writing continues, in overwriting of another track or in the data not being found when it is to be read. Most conventional tape systems, therefore, are designed to react quickly when there is an xe2x80x9coff-track error,xe2x80x9d that is, when the head reaches an xe2x80x9coff-track limitxe2x80x9d that represents a certain distance from the center of the track. This off-track limit is predetermined, by the manufacturer/programmer, to be the limit between the area in which writing may be done properly and the area in which writing is not acceptable because of the possibility/probability of overwriting or unreadable writing. The off-track limit, in terms of microns transversely off of center-track, is set at different locations by various manufacturers, preferably in the range of about 10-20% of the track width. For a track having a 28-micron width, the off-track error limit may be set, for example, at about 3 microns, or another limit within the preferred 10-20% range.
The off-track sensing system of a conventional tape system cooperates with the closed-loop head-positioning servo control system. The head monitors its position relative to the servo bands, and so, in effect, monitors the position of the writing elements relative to the centers of the tracks. When the head reaches the off-track limit, meaning that the individual elements are an off-track relative to their respective track centers, an error is signaled. The conventional tape system controller then turns off the writing function while the tape continues to travel at the normal maximum speed, and waits until the head is back xe2x80x9con track,xe2x80x9d that is, within the acceptable position limits. Once the sensing system indicates that there is no longer an off-track error, writing is started again.
Each time an off-track error occurs, the process is repeated. If there are tape, reel, tape wrapping, or vibration problems inherent in the tape system or in the external environment, off-track errors may repeat frequently. In such a case, the conventional tape system repeatedly xe2x80x9cretriesxe2x80x9d the writing, that is, shuts off the writing function, advances the tape until the off-track error ends, and starts writing again until another off-track error signal occurs, all at the normal, maximum tape speed. Optionally, conventional retries may include reversing the tape to backup to a position on the tape where writing stopped, but such methods add complexity and waste time. If repeated off-track errors continue to interrupt writing for an extended period of time, the tape may travel a long distance without a significant amount of successful writing. Therefore, to prevent endless unsuccessful attempts at writing, the conventional tape controller stops this retry process if a certain amount of writing has not been done successfully within a certain length of tape. Typically, if a data-set, which is the smallest block of data to be written (typically 100-110 mm of writing), has not been written in a certain total length of tape (typically 4 meters), the controller will stop the process, without any more retries, and signal a write failure.
Some tape speed control systems for linear tape have been developed that may adjust tape speed in order to match the speed of data coming from the host computer. Such systems prevent frequent stops and starts of the tape writing process when the tape speed is ahead of the data delivery speed from the host computer. In systems using such a host-matching speed control system, the speed at which writing, tape advancement, and retries are conducted is the xe2x80x9cmaximum target speed,xe2x80x9d which is set in response to the host data transfer rate. In such systems, therefore, the target speed may not be the system""s highest possible speed, but rather a speed set as a function of the host computer""s operation. Patents that discuss speed control include U.S. Pat. No. 5,892,633 (Ayres, et al.); U.S. Pat. No. 5,764,430 (Ottesen, et al.); and U.S. Pat. No. 6,067,203 (Ottesen, et al.).
Still, there is a need for improved tape writing and reading performance. There is a need for off-track error reduction and improved methods for correction/control of off-track errors. The present invention addresses these needs.
The present invention comprises a method of reducing the negative effects of off-track errors in data storage tape systems, by helping to limit the frequency of off-track errors. The present invention reduces tape speed in response to an off-track error during writing of a data-set on a tape, which action tends to correct the error and reduce the recurrence of off-track errors. Overall, the present invention tends to increase successful writing and to reduce failed writing.
Additionally, the invention may comprise apparatus for implementing the preferred methods, comprising a system for controlling tape speed in response to off-track errors. In some embodiments, the controller stops writing in response to an off-track error, reduces tape speed incrementally, and then resumes writing after the incremental reduction, assuming the writing head has gone back on-track. In especially-preferred embodiments, the controller stops writing in response to an off-track error, reduces tape speed continuously, and resumes writing as soon as the head goes back on-track, which is normally during the speed reduction. The invention may also comprise methods and apparatus for accelerating tape speed at desired times or after desired events, for example, after a data-set is successfully written.